We have three methods of VBUS detection:

- GPIO (VBUS connected directly to GPIO, GPIO interrupt routine detects)
- Charger (Charger IC asserts interrupt pin, wakes USB_CHG task, USB_CHG task detects)
- TCPC (TCPC asserts alert interrupt pin, wakes PD_CMD task, calls tcpc_alert() which detects)

All three methods are interrupt-based, but the second two involve other tasks, because checking VBUS level involves i2c.

When a VBUS change occurs, other modules / tasks need to be informed:

- PD tasks need to be told VBUS went low (record keeping of PD_FLAGS_VBUS_NEVER_LOW)
- Corresponding PD task needs to be woken.
- Corresponding USB charger task needs to be woken.
- charge_manager needs to be told of the change.

The code to do this informing is spread out, and in some cases incomplete or completely missing. Ideally we should have a single common routine that takes all of these actions, which will be called in a similar way regardless of the VBUS detect method used. Let's try to implement that.